In general, a machine structural member, such as an automobile drive shaft or an automobile constant velocity joint, is provided with high fatigue strength, such as torsional fatigue strength, bending fatigue strength, and roller pitting fatigue strength, which are all important characteristics for the machine structural member, by working a hot rolled steel bar into a member with a predetermined shape through hot forging, cutting, cold forging and the like, followed by induction hardening and tempering.
In recent years, further improvement in the fatigue strength of such a machine structural member has been required as the demand for weight reduction of automobile members increases in view of environmental issues.
Various methods have been proposed to improve the fatigue strength. For example, increasing the depth of induction hardening may be expected to improve the torsional fatigue strength. However, the torsional fatigue strength levels off at a certain depth, no further improvement being realized.
Increasing grain boundary strength is also effective in improving the torsional fatigue strength. For example, Japanese Unexamined Patent Application Publication No. 2000-154819 discloses a method for decreasing the austenite grain size by dispersing a large amount of fine TiC during induction heating.
However, the dispersion of a large amount of fine TiC during induction heating requires that the steel should be heated to at least 1100° C. for solution treatment of TiC in advance. This results in low productivity. Furthermore, only decreasing the austenite grain size by the dispersion of a large amount of fine TiC is not sufficient to satisfy the recent demand for torsional fatigue strength.
Japanese Unexamined Patent Application Publication No. 8-53714 discloses a machine structural component with improved torsional fatigue strength, in which a value A that is calculated from CD/R, γf, Hf, and Hc is adjusted within a predetermined range depending on the C content, wherein CD/R is the ratio of the thickness (case depth) CD of a hardened layer formed by induction hardening of the machine structural component having a circular cross section to the radius R of the circular cross section, the CD/R being limited to 0.3-0.7; γf is prior austenite grain size through the depth up to 1 mm in the induction hardened layer; Hf is average Vickers hardness of an as-quenched component in the CD/R range up to 0.1; and Hc is average Vickers hardness at an axial center after the induction hardening.
However, since the prior austenite grain size through the depth of the hardened layer is not taken into consideration in this machine structural component, the recent demand for torsional fatigue strength is not satisfied, either.
It could therefore be advantageous to provide a steel product for induction hardening which allows higher fatigue strength than ever before after induction hardening, and an induction hardened member with high fatigue strength prepared from the steel product. It could also be advantageous to provide a method for manufacturing the steel product and the induction hardened member.